This invention relates generally to a method of treating a ferrous strand, such as continuous strip or wire, and controlling the coating conditions to eliminate coating defects such as pinhole and bare spots, prior to the entry of said strand into a molten pot of an aluminum-zinc alloy. Thus, while this invention is applicable to the treatment of strip and wire, the more observable results are seen in the production of a continuous strip. Accordingly, the further description shall be directed to the treatment of such a strip. However, this should not be read as a limitation on the invention.
Ferrous articles coated with a metallic layer of non-ferrous metal have long been utilized in mildly corrosive environments to great advantage. Typically, structural stability comes from the ferrous core while the coating affords resistance to corrosive attack. Optimum resistance is achieved where the protective layer is uniform and continuous. From this it would follow that a clean ferrous strip and effective coating conditions are essential for optimum corrosion resistance.
The prior art, particularly in the hot dip coating of aluminum on steel, early recognized the need to provide a clean, essentially oxide-free, strip prior to its entry into the molten bath. The aluminum would not adhere where oxide was present on the surface; hence, bare spots appeared. Further, bath surface oxidation at the point of entry for the strip has resulted in the defects enumerated. Elaborate procedures were devised in an attempt to avoid these undesirable results. Unfortunately they were not always effective, for even the slightest amount of moisture at the point of strip entry into the bath resulted in bare spots and/or pin holes, evidencing the presence of surface oxide.
The patent to Coburn, U.S. Pat. No. 3,051,587, represents one such attempt by the prior art. The patentee teaches a procedure for entraining the ferrous strip, just prior to its entry into the molten aluminum bath, in an atmosphere of sodium as a means to scavenge oxygen and water vapor in the strip entry snout. Teshima et al., U.S. Pat. No. 3,027,269, proposed to solve the problem of voids and pin holes with the application of a thin layer of an aqueous solution consisting of an alkali metal compound to the ferrous strip, drying in situ, and subjecting the thus coated strip to a reducing atmosphere prior to immersing in a bath of aluminum.
However, neither of these procedures were totally effective. Due to the very strong affinity of aluminum for oxygen, even the most minute quantities of oxygen or water vapor affected at least the bath surface, and the strip passing therethrough.
By the system of the present invention, an effective means is taught for the production of a ferrous strand having on its surfaces a uniform coating of about 25% to 70%, by weight, aluminum, balance essentially zinc, free of pin hole and bare spot defects.